1. Field of the Invention
The present invention relates to a photocurrent sensing circuit converting an illumination of visible light into an electric signal as well as a photosensor and an electronic device that are provided with the photocurrent sensing circuit, and particularly to a photocurrent sensing circuit that has spectral sensitivity characteristics close to human visual sensitivity characteristics as well as a photosensor and an electronic device provided with the same.
2. Description of the Background Art
As typical examples of photosensors for visible light, there are two kinds of sensors of silicon photodiodes and CdS (Cadmium Sulfide) cells are. Since the silicon photodiode has small sizes, high responsibility and high stability, it is widely used in light receiving elements for optical communications and optical disks as well as photosensors and others. However, spectral sensitivity characteristics of the silicon photodiode significantly differ from human visual sensitivity characteristics in that the sensitivity in the infrared range is high. Conversely, the CdS cell has the spectral sensitivity characteristics close to the human visual sensitivity characteristics. Therefore, the CdS cells have been used in exposure meters of cameras and visual light sensors for long years.
In recent years, there has been a problem relating to use and the like of substances causing high environmental loads, and restrictions are being introduced on use of the CdS cells containing cadmium sulfide as a major component. For example, Europe has inhibited the bringing in of products using cadmium, lead, hexavalent chromium or mercury since July 2006. Conversely, the silicon causes a less environmental load than the cadmium sulfide. Therefore, it has been increasingly demanded to provide a sensor that is formed of a silicon photodiode, and has the spectral sensitivity characteristics close to the human visual sensitivity characteristics.
In recent years, it has been increasingly demanded to provide illumination sensors that have the spectral sensitivity characteristics close to the human visual sensitivity characteristics for using them as sensors that automatically adjust a brightness of a backlight such as a cellular phone, a LCTV (Liquid Crystal Television) or the like according to an ambient brightness for suppressing power consumption of a battery of the cellular phone and improving visibility of a liquid crystal display.
For suppressing exhaustion of a battery, it is also necessary to reduce power consumption of an illumination sensor itself In general, the output of the illumination sensor is proportional to a quantity of incident light. When light of a high illumination such as direct sunlight illumination enters the sensor, it outputs a current of several milliamperes or more, and this affects a battery life. In recent years, therefore, the cellular phones and mobile terminals have employed a logarithmic current output type of illumination sensors that include a circuit performing logarithmic transformation of an incident light quantity (e.g., see Japanese Patent Laying-Open Nos. 08-065074, 08-340128, 10-021314 and 2006-133942) and thereby suppress current consumption over a wide range from low to high illuminations. For finely controlling backlight illuminating of the liquid crystal panel, it is also required to achieve accurate output of the illumination sensor and to improve the temperature characteristics thereof.
FIG. 7 is a circuit diagram showing a structure of a conventional photocurrent sensing circuit.
Referring to FIG. 7, a photocurrent sensing circuit 301 includes a light receiving element PD, PNP transistors QP51-QP56, NPN transistors QN51-QN56, diodes D51 and D52, constant current supply circuits IS51-IS54, resistances RE and Rout, and an output terminal T51. NPN transistors QN51 and QN52, resistance RE and constant current supply circuits IS51 and IS52 form a transconductance amplifier 61.
Photocurrent sensing circuit 301 performs logarithmic transformation on the photocurrent of light receiving element PD to provide an output current Iout. More specifically, when light receiving element PD receives the light, a photocurrent Ipd proportional to the quantity of received light flows through light receiving element PD. Diode D51 coverts it into a logarithmically compressed voltage. A voltage VD1 between the opposite ends of diode D51 is expressed by the following equation:VD1=Vt×ln(Ipd/Is)where Vt represents a thermal voltage of diode D51 expressed by (k×T/q), k represents a Boltzmann's constant, T represents an absolute temperature, q represents an elementary charge, i.e., quantum of electricity and Is represents a reverse saturation current of diode D51. ln represents a natural logarithm.
Diode D52 converts a reference current Iref provided from constant current supply circuit IS53 into a logarithmically compressed voltage. A voltage VD2 between the opposite ends of diode D52 is expressed by the following equation:VD2=Vt×ln(Iref/Is)
Voltages VD1 and VD2 are provided to bases of NPN transistors QN51 and QN52 of transconductance amplifier 61, respectively. The bases of NPN transistors QN51 and QN52 provide differential inputs to transconductance amplifier 61. A voltage difference ΔVD between voltages VD1 and VD2 is expressed by the following equation:
                              Δ          ⁢                                          ⁢          VD                =                              VD            ⁢                                                  ⁢            1                    -                      VD            ⁢                                                  ⁢            2                                                  =                              Vt            ×                          ln              ⁡                              (                                  Ipd                  /                  Is                                )                                              -                      Vt            ×                          ln              ⁡                              (                                  Iref                  /                  Is                                )                                                                            =                  Vt          ×                      ln            ⁡                          (                              Ipd                /                Iref                            )                                          
Transconductance amplifier 61 converts voltage difference ΔVD into a current. A transconductance gm of transconductance amplifier 61 is expressed by the following equation, where RE represents a resistance value of resistance RE, and Io represents an output current value of constant current supply circuits IS51 and IS52.gm=1/(2×Vt/Io+RE)
Therefore, a collector current I51 of NPN transistor QN52 is expressed by the following equation:
                              I          ⁢                                          ⁢          51                =                  Io          +                      gm            ×            Δ            ⁢                                                  ⁢            VD                                                  =                  Io          +                      Vt            ×                                          ln                ⁡                                  (                                      Ipd                    /                    Iref                                    )                                            /                              (                                                      2                    ×                                          Vt                      /                      Io                                                        +                  RE                                )                                                        
When (2×Vt/Io<<RE) is satisfied, collector current I51 of NPN transistor QN52 is expressed by the following equation:I51≈Io+Vt×ln(Ipd/Iref)/RE 
A current mirror circuit formed of PNP transistors QP52 and QP54 as well as a current mirror circuit formed of NPN transistors QN55 and QN56 change the current direction of collector current I51 of NPN transistor QN52, i.e., an output current I1 of the transconductance amplifier, and an output current Ib of constant current supply circuit IS54 having the same temperature coefficient as output current Io of constant current supply circuits IS51 and IS52 is subtracted from this collector current I51. Thereby, collector current I52 of PNP transistor QP55 is expressed by the following formula:
                              I          ⁢                                          ⁢          52                =                              I            ⁢                                                  ⁢            51                    -          Ib                                        =                              I            ⁢                                                  ⁢            0                    -          Ib          +                      Vt            ×                                          ln                ⁡                                  (                                      Ipd                    /                    Iref                                    )                                            /              RE                                          
When PNP transistors QP55 and QP56 have the same emitter areas, output current Iout of photocurrent sensing circuit 301 is expressed by the following equation:
                    Iout        =                  I          ⁢                                          ⁢          52                                        =                  Io          -          Ib          +                      Vt            ×                                          ln                ⁡                                  (                                      Ipd                    /                    Iref                                    )                                            /              RE                                          
For providing output current Iout of photocurrent sensing circuit 301 equal to (10×log(illumination)), output current Ib of constant current supply circuit IS54 is determined in the following manner, where log represents a logarithmic using base 10.
When the illumination is 1 lux, output current Iout is expressed by the following equation where Ipd_1lx represents photocurrent Ipd of light receiving element PD.Iout=Io−Ib+(Vt/RE)×((ln(Ipd/Ipd—1lx)+ln(Ipd—1lx/Iref))
The transformation equation of the base of ln and the base of log is as follows:ln(X)=log(X)/log(e)≈2.3025×log(X)
Therefore, output current Iout is expressed by the following formula:Iout=Io−lb+(Vt/RE)×(2.3025×log(Ipd/Ipd—1lx)−ln(Iref/Ipd—1lx))
Assuming that Ev represents the illumination of the incident light of light receiving element PD, a relationship of (Ev=Ipd/Ipd_1lx) is satisfied so that output current Iout is expressed by the following equation:
                    Iout        =                ⁢                  Io          -          Ib          +                                    (                              Vt                /                RE                            )                        ×                          (                                                2.3025                  ×                                      log                    ⁡                                          (                      Ev                      )                                                                      -                                  ln                  ⁡                                      (                                                                  Iref                        /                        Ipd_                                            ⁢                      1                      ⁢                      lx                                        )                                                              )                                                              =                ⁢                              (                          Vt              /              RE                        )                    ×                      (                                          2.3025                ×                                  log                  ⁡                                      (                    Ev                    )                                                              -                              ln                ⁡                                  (                                                            Iref                      /                      Ipd_                                        ⁢                    1                    ⁢                    lx                                    )                                            +                                                                      ⁢                              (                          RE              /              Vt                        )                    ×                      (                          Io              -              Ib                        )                          )            
When Io, Ib and RE are appropriately determined to satisfy the relationships of:(RE/Vt)×(Io−lb)−ln(Iref/Ipd—1lx)=0, and(Vt/RE)×2.3025=10output current Iout is expressed by the following equation:Iout=10×log(Ev)
Therefore, output current Iout can be set ten times larger than the logarithmic value of the illumination.
Constant current supply circuits IS51-IS54 are band gap current supplies, respectively, and reference current Iref and output currents Io and Ib are expressed by the following equations, respectively.I=Vt×ln(10)/R 
Temperature coefficients of thermal voltage Vt and a resistance value R may be equal to each other, whereby temperature-changes in output currents Io and Ib can be small.
Output current Iout is expressed by the following equation:Iout=N×(Io−Ib+Vt×ln(Ipd/Iref)/RE)where N represents an amplification factor of PNP transistor QP56.
Therefore, by employing such a configuration that the temperature coefficient of resistance value RE is equal to that of thermal voltage Vt of diode D51, the temperature-changes in output current Iout can be small.
A temperature coefficient α of thermal voltage Vt of diode D51 is expressed by the following equation:α=(∂Vt/∂T)/Vt 
Since Vt is equal to kT/q (Vt=kT/q), α is expressed by the following equation:α=1/T 
When the temperature is, e.g., 27° C.(=300 (K) in absolute temperature T), temperature coefficient α of thermal voltage Vt is equal to 3333 ppm/deg.C. Even when the temperature coefficient of resistance RE at 27° C. is set to 3333 ppm/deg.C., the temperature may deviates from 27° C., and resistance RE was produced using an ordinary semiconductor process. In this case, a difference occurs in temperature coefficient between thermal voltage Vt and resistance RE so that temperature-changes in output current Iout occur. Further, for providing thermal voltage Vt and resistance RE having the temperature coefficients equal to each other, it is necessary to integrate a special temperature measuring resistance having the same temperature coefficient as thermal voltage Vt over a wide temperature range, which increases a manufacturing cost of the photocurrent sensing circuit.